1. Field of the Invention
This invention relates to a novel coating for use as a protective coating for ceramic materials such as ceramic fabrics made of silicon carbide, aluminum oxide, zirconium oxide, aluminoborosilicate and silicon dioxide. This invention also relates to a re-usable thermal control structure comprising a ceramic material as a s substrate having coated thereon the protective coating of this invention which provides excellent resistance to high temperatures and repeated thermal shock at temperatures up to 1650.degree. C. In another aspect of this invention, the protective coating may be coated on a flexible ceramic fabric, which ceramic fabric is the outer cover of a composite insulation. In yet another aspect, the protective coating of this invention can also be utilized as a high temperature adhesive to bond metallic foils to ceramic fabrics and composite insulations. A primary application of this invention is as a protective coating for ceramic materials used in a heat shield for space vehicles subjected to very high aeroconvective heating environments.
To retain the thermal control structure for reuse capability under such space and atmospheric conditions, it was necessary to develop a high density protective surface coating. The requirements for this protective coating included a match of the thermal expansion with the substrate material and imperviousness to moisture, resistance to abrasion and mechanical damage, high emissivity at high temperatures in the critical wavelengths below 3 .mu.m, capability of surviving cyclic heating to 1650.degree. C., and good thermal shock, vibration and acoustic performance. Furthermore, the protective coating has to be applied thinly in order to retain the flexibility of a coated ceramic fabric and to avoid o excessive weight gain.
The present invention provides increased total hemispherical emissivity and decreased catalytic efficiency of the thermal control structure surface when subjected to a high temperature environment. Particularly, the protective coating of this invention lowers the surface temperature of the thermal control structure and reduces heat transfer through the surface so that the underlying ceramic material is protected from degradation.
2. Description of the Prior Art
Protective coatings suitable for use on ceramic insulations are known in the art. D. Mui and H. M. Clancy in "Development of a Protective Ceramic Coating for Shuttle Orbiter Advanced Flexible Reusable Surface Insulation (AFRSI)", Ceramic Engineering and Science Proceedings, Vol. 6, No. 7-8, pp. 793-805 (July-August 1985), disclose such compositions and structures which were proven to be successful on Space Shuttle flights. While these glass coatings have good physical and optical properties in the convective heating environment encountered during reentry, they are limited as follows: (1) The protective coating of Mui and Clancy provides adequate protection only up to 1800.degree. F. (982.degree. C). (2) When heated above this temperature, cristobalite formation may result in a cracked coating. (3) The emittance of the coating is close to that of pure silicon dioxide, such that the coating does not enhance the emittance of the underlying fabric. Therefore, the temperature capability of the system is not increased.
The above described prior art coating typically contains two ingredients of known high purity, namely ammonia-stabilized colloidal silicon dioxide in deionized water, and ground silicon dioxide. The coating is easily applied on ceramic fabrics and insulations such as the Advanced Flexible Reusable Surface Insulation (AFRSI) used on the Space Shuttle Orbiter. This prior art coating can not be used on reentry space vehicles such as the Advanced Space Transfer Vehicle (ASTV), since it is unstable at the high temperatures (e.g., 1600 .degree. C.) which would be produced during reentry of these vehicles. The ASTV is described in Walberg, G. D., "A Survey of Aeroassisted Orbit Transfer", J. Spacecraft & Rockets, Vol. 22, No. 1 (January-February 1985).
On the other hand, the RCG (reaction cured glass) high temperature coating described in U.S. Pat. No. 4,093,771 was designed primarily for rigid ceramic insulations and is not suitable for flexible ceramic fabrics and insulations which must remain flexible during installation and reentry. In order to be effective as a protective coating, the above noted prior art coating must be fired for a minimum firing time of one hour. When this coating having a final weight of 0.07 g/cm.sup.2 is fired on a ceramic fabric, the fabric becomes extremely fragile and non-flexible. This coating also requires an organic binder (e.g., methylcellulose) to hold itself together before firing. The organic binder will volatilize and coat adjacent surfaces in a space vehicle when the unfired coating is first heated.
The protective coating of U S. Pat. No. 4,093,771 typically contains two ingredients of known high purity, namely, borosilicate glass and silicon tetraboride. The coating is applied only on rigid ceramic insulations such as Fibrous Reusable Composite Insulation (FRCI) used on the Space Shuttle Orbiter. Furthermore, this coating relies on the oxidation of its emittance agent (i.e., silicon tetraboride) and the fluxing of the glass by the boron oxide to seal the coating and to prevent further oxidation. As discussed below, the present invention does not rely on this mechanism to seal the glass and therefore does not require firing prior to use.
Other coatings known in the art are the two layer coatings described in U.S. Pat. No. 4,381,333. These two layer coatings oonsist of a base Coating such as that described in U.S. Pat. No. 4,093,771 and a reflective surface overlay. The reflective surface overlay consists of fused silicon dioxide, borosilicate glass, and an organic carrier solution. These coatings, which are used primarily on rigid ceramic insulations, must be fired before use and suffer from the same limitations as discussed above when applied to flexible ceramic fabrics and insulations.
Thus, while the art of protective coatings for ceramic insulations is a well developed one, a need remains for further development of a lightweight surface coating especially adapted for use on heat shields for space vehicles subjected to extreme heating environments.